In recent years, energy use has become more efficient, and demand has emerged for a reduction in energy loss at the time of operation, for example in a transformer.
The loss occurring in a transformer is mainly composed of copper loss occurring in conducting wires and iron loss occurring in the iron core.
Iron loss can be further divided into hysteresis loss and eddy current loss. To reduce the former, measures such as improving the crystal orientation of the material and reducing impurities have proven effective. For example, JP 2012-1741 A (PTL 1) discloses a method for manufacturing a grain-oriented electrical steel sheet with excellent flux density and iron loss properties by optimizing the annealing conditions before final cold rolling.
On the other hand, in addition to reducing sheet thickness and increasing the added amount of Si, the eddy current loss is also known to improve dramatically by the formation of a groove or the introduction of strain on the surface of the steel sheet.
For example, JP H06-22179 B2 (PTL 2) discloses a technique for forming a linear groove, with a groove width of 300 μm or less and a groove depth of 100 μm or less, on one surface of a steel sheet so as to reduce the iron loss W17/50, which was 0.80 W/kg or more before groove formation, to 0.70 W/kg or less.
JP 2011-246782 A (PTL 3) discloses a technique for irradiating a secondary recrystallized steel sheet with a plasma arc so as to reduce the iron loss W17/50, which was 0.80 W/kg or more before irradiation, to 0.65 W/kg or less.
Furthermore, JP 2012-52230 A (PTL 4) discloses a technique for obtaining material for a transformer with low iron loss and little noise by optimizing the coating thickness and the average width of a magnetic domain discontinuous portion formed on the surface of a steel sheet by electron beam irradiation.
It is known, however, that the iron loss reduction effect achieved by such groove formation or introduction of strain differs depending on the sheet thickness of the material. For example, IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-20, NO. 5, p. 1557 (NPL 1) describes how, as the sheet thickness increases, the amount of reduction in iron loss due to laser irradiation tends to decrease and notes a difference of approximately 0.05 W/kg in the amount of reduction in iron loss (ΔW17/50) between sheet thicknesses of 0.23 mm and 0.30 mm for a material with a flux density of 1.94 T.
Against this background, studies have been made of whether the effect of reducing iron loss of thick sheet material can be improved even slightly by adjusting the magnetic domain refining method. For example, JP 2000-328139 A (PTL 5) and JP 4705382 B2 (PTL 6) disclose techniques for improving the effect of reducing iron loss of a grain-oriented electrical steel sheet from thick sheet material by optimizing the laser irradiation conditions in accordance with the sheet thickness of the material. In particular, PTL 6 discloses having obtained extremely low iron loss by setting the strain ratio η to 0.00013 or more and 0.013 or less.
This strain ratio η is the ratio of the strain area within a rolling direction cross-section of the steel sheet and is expressed by the formula η/8×(w×w)/(t×PL), where t is the thickness of the steel sheet, w is the closure domain width in the rolling direction, and PL is the laser irradiation spacing in the rolling direction.